Light emitting devices and particularly vertical-cavity surface-emitting lasers (VCSELs) are useful in many communication applications, such as, for example, optical fiber communication. VCSELs generally include a light emitting region (commonly referred to as an “active region”) located between a pair of DBRs fabricated on a semiconductor substrate. The VCSEL also includes various buffer layers, optical confinement regions, current confinement regions and electrical conductors. Light is emitted from the active region, reflected by the DBRs and emitted from one surface of the VCSEL. Generally, depending on the desired application, a VCSEL can emit light at either relatively short wavelengths (i.e., on the order of 850 nanometers (nm)) or relatively long wavelengths (i.e., on the order of 1300-1600 nm). Most optical communications applications require that the emitted light be at relatively long wavelengths. Furthermore, it is desirable to fabricate the DBRs, active region, buffer layers, and the optical and current confinement regions during a single epitaxial growth. Generally, active regions capable of emitting light at relatively long wavelengths can be fabricated using the indium phosphide (InP) material system.
Unfortunately, single epitaxial growth, long wavelength VCSELs are difficult to fabricate in the InP material system because of the difficulty in fabricating high quality DBRs having a high index contrast.
High index contrast DBRs can be fabricated in the gallium arsenide (GaAs) material system, but fabricating an active region that is capable of emitting long wavelength light in the GaAs material system has proven difficult.
When using the InP material system to fabricate a VCSEL, the available material choices for DBRs are limited to indium gallium arsenide phosphide/indium phosphide (InGaAsP/InP) or indium aluminum arsenide/indium aluminum gallium arsenide (InAlAs/InAlGaAs), both of which require lattice matching to the substrate material. These compositions also exhibit a low index contrast, thereby requiring many pairs of layers to obtain the desired 99.9% reflectivity and both exhibit a relatively narrow stopband width. Indeed, InGaAsP/InP and InAlAs/InAlGaAs DBRs may require as many as 65 layer pairs to obtain the desired 99.9% reflectivity. DBRs fabricated using aluminum arsenide antimonide/aluminum gallium arsenide antimonide (AlAsSb/AlGaAsSb) exhibit a somewhat higher refractive index contrast, but still require lattice matching and a relatively large number of layer pairs.
One type of DBR that has the desired reflectivity, a low number of layer pairs and a high stopband width is known as an air/semiconductor DBR. An air/semiconductor DBR exhibits a high index contrast between the layers because the semiconductor layers are separated by air. In such a DBR reflectivity of >99.9% can be obtained using only three pairs of air/semiconductor layers.
An air/semiconductor DBR is fabricated by growing or depositing epitaxial films that have a high etch selectivity, such as InGaAs and InP. For example, nearly complete etch selectivity can be obtained if the InGaAs material is used as a “sacrificial” layer that is etched by a solution of ferric chloride (FeCi3:H2O). In such a DBR, the layers are grown or deposited and then etched to form a mesa exposing the layers. A suitable etch mask, such as silicon dioxide (SiO2) is deposited and patterned, exposing portions of the mesa where the airgap DBR is to be defined. The sacrificial InGaAs layer(s) is removed by the ferric chloride etch resulting in a suspended airgap DBR supported by the unetched regions.
Unfortunately, such a DBR suffers from poor mechanical stability. The airgap DBR may buckle due to residual stress in the epitaxial layers or may collapse due to capillary forces created during the etching, rinsing and drying processes. Furthermore, conventional airgap DBRs are limited in size and the thickness of the layers is difficult to precisely control because of the above-mentioned deficiencies.
Therefore, there is a need in the industry for a vertical-cavity surface-emitting laser (VSCEL) including a mechanically stable airgap DBR that can be easily and economically fabricated with a high degree of precision.